Sulfate injection to stop global warming?

Apparently, Paul Crutzen, an environmental scientist who shared a Nobel Prize in 1995 for his work on the role of CFCs in ozone layer depletion, thinks we should correct for global warming by injecting two million tonnes per year of sulphate particles into the upper atmosphere. According to Wikipedia: “sulfates occur as microscopic particles (aerosols) resulting from fossil fuel and biomass combustion. They increase the acidity of the atmosphere and form acid rain.” He predicts that the process of injecting them into the upper atmosphere using balloons or artillery would cost between $25 and $50 billion a year, but would save more by mitigating the effects of global warming.

While I am no environmental scientist, what strikes me as most interesting about this is the ‘technical fix’ mindset that it embodies: a bit like those who decided to stabilize dune formation on parts of the Oregon coast by importing Spanish beach grass, or those who have sought to kill off one accidentally imported pest with an intentionally imported predator. Often, such schemes don’t work at all. When they do, they risk working much too well. Thanks to Spanish beach grass, the Oregon dunes will be a thing of the past in a few decades. The point is simply that, at a stage when we really don’t know the consequences of climate change or their magnitude, it seems awfully bold to predict that such a scheme will both work and do more good than harm.

As is so often the case, the most trenchant criticism of such schemes was expressed humorously on The Simpsons:

SKINNER: Well, I was wrong. The lizards are a godsend.

LISA: But isn’t that a bit short-sighted? What happens when we’re overrun by lizards?

SKINNER: No problem. We simply unleash wave after wave of Chinese needle snakes. They’ll wipe out the lizards.

LISA: But aren’t the snakes even worse?

SKINNER: Yes, but we’re prepared for that. We’ve lined up a fabulous type of gorilla that thrives on snake meat.

The comparison between atmospheric science and ecology is less dubious than one might think. Both systems are complex and dynamic – they feed back upon themselves in ways which are both powerful and difficult to predict. Furthermore, both atmospheric and ecological systems both affect and are affected by other complex systems with which they are integrated. Consider, for instance, how the construction of the Aswan High Dam (the product of political and economic changes, above all) altered the salinity in the eastern Mediterranean, allowing for the migration of species from the Red Sea.

What would the consequences of blasting artillery shells full of sulfates into the upper atmosphere? Far be it for me to speculate. The intentional modification of atmospheric chemistry and physics is something we have never done as a species, though we have done a lot of unintentional tinkering. What I would venture is that it is likely to have unpredictable effects and that it is a particularly curious way of trying to deal with the problem of global warming.

George Monbiot, who I met at a short conference at the Environmental Change Centre, has his own objections.

FEW scientists like to say so, but cutting greenhouse-gas emissions is not the only way to solve the problem of global warming. If man-made technologies are capable of heating the planet, they are probably capable of cooling it down again. Welcome to “geo-engineering”, which holds that, rather than trying to change mankind’s industrial habits, it is more efficient to counter the effects, using planetary-scale engineering.

The problem of global warming arises from the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine by emulating a volcanic eruption. In between the incoming solar radiation and the OLR is the entire weather and climate system and the hydrological cycle. The precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from El Chichón in March 1982 and Pinatubo in June 1991, taking into account changes from El Niño-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991-September 1992. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions.

“We support efforts to find ways of sequestering carbon, but the likely consequences of geo-engineering schemes should be thoroughly researched before they are promoted as solutions. We do not consider ocean fertilization to be a promising approach, and on a large scale it would constitute major interference with an ecosystem which is still poorly understood. Fertilization is likely to alter the phytoplankton community composition and succession, and thus the structure of the oceans’ food webs. It might damage these remote and possibly fragile ecosystems, trigger unexpected feedbacks and even reduce their ability to sequester carbon. We cannot, therefore, support this approach, until it can be shown that there would be demonstrable benefits which would outweigh the potential impacts.”

“Also, a planet with a dim Sun and high CO2 is not the same thermodynamically as a planet with brighter Sun and lower CO2, because the reduced sunlight at the surface is not able to sustain as much evaporation, which has consequences for global rainfall. In a recent essay in Le Monde, Edouard Bard has pointed out additional problems with geoengineering.

In my mind, the most serious peril of sulfate geoengineering is one that stems from a problem that is not at all in dispute: the fact that the lifetime of CO2 in the atmosphere is centuries to millennia, whereas the lifetime of aerosols in the stratosphere is at best a few years. That means committing the future generations to continue the aerosol injection basically every year more or less forever. We’re banking a lot on confidence in future stability and prosperity of the world here. A patrician in the glory days of the Roman Empire might well have expected the Pax Romana to go on forever, but really nobody expects a Dark Age.”

The purpose of this report is to imagine the unthinkable – to push the boundaries of current research on climate change so we may better understand the potential implications on United States national security.

We have interviewed leading climate change scientists, conducted additional research, and reviewed several iterations of the scenario with these experts. The scientists support this project, but caution that the scenario depicted is extreme in two fundamental ways. First, they suggest the occurrences we outline would most likely happen in a few regions, rather than on globally. Second, they say the magnitude of the event may be considerably smaller.

We have created a climate change scenario that although not the most likely, is plausible, and would challenge United States national security in ways that should be considered immediately.

The large burden of sulfate aerosols injected into the stratosphere by the eruption of Mount Pinatubo in 1991 cooled Earth and enhanced the destruction of polar ozone in the subsequent few years. The continuous injection of sulfur into the stratosphere has been suggested as a “geoengineering” scheme to counteract global warming. We use an empirical relationship between ozone depletion and chlorine activation to estimate how this approach might influence polar ozone. An injection of sulfur large enough to compensate for surface warming caused by the doubling of atmospheric CO2 would strongly increase the extent of Arctic ozone depletion during the present century for cold winters and would cause a considerable delay, between 30 and 70 years, in the expected recovery of the Antarctic ozone hole.

Robock et al used a coupled GCM with interactive aerosols to see what would happen if they injected huge amounts of SO2 (the precursor of sulphate aerosols) into the tropical or Arctic stratosphere. This is the most talked about (and most feasible) geoengineering idea, based on the cooling impacts of large tropical volcanic eruptions (like Mt. Pinatubo in 1991). Bottom line? This is no panacea.

So what are the problems? Robock’s study looks at a subset of the potential ones – in particular, the impacts on precipitation. These arise because evaporation is more sensitive to changes in solar radiation than it is to long-wave radiation – so increasing LW and decreasing SW (as you would have in a geo-engineered future) gives a net reduction in evaporation even if the temperatures stay pretty constant. In the experiments they report on, there is a substantial reduction in rainfall in the northern tropics (especially the Sahel and the monsoonal belts). This is actually quite a robust result: reductions in tropical precipitation were reported in simpler tests of this idea in papers by Matthews and Caldiera and Bala et al…

From the Technical Summary of the Working Group I section of the Fourth Assessment Report of the IPCC:

‘Global dimming’ is not global in extent and it has not continued after 1990. Reported decreases in solar radiation at the Earth’s surface from 1970 to 1990 have an urban bias. Further, there have been increases since about 1990. An increasing aerosol load due to human activities decreases regional air quality and the amount of solar radiation reaching the Earth’s surface. In some areas, such as Eastern Europe, recent observations of a reversal in the sign of this effect link changes in solar radiation to concurrent air quality improvements.

Sep 4th 2008
From The Economist print edition
Some scientists think climate change needs a more radical approach. As well as trying to curb greenhouse-gas emissions, they have plans to re-engineer the Earth

…

Perhaps the most intriguing idea—which was published last year, though not discussed by the Royal Society—is to eject carbon dioxide from the atmosphere at the Earth’s poles, using the planet’s magnetic field. This may sound absurd, but oxygen already leaks out this way (the phenomenon is the subject of a paper just published by Hans Nilsson of Swedish Institute of Space Physics). Alfred Wong, a researcher at the University of California, Los Angeles, proposes that a system involving powerful lasers and finely tuned radio waves could encourage carbon dioxide to take the same route. His calculations suggested that using lasers to ionise molecules of carbon dioxide, and radio waves to get them to spin at the correct rate, would cause those molecules to spiral away from Earth along the lines of magnetic force until they were lost for ever in space.

Jan 29th 2009
From The Economist print edition
Plans to engineer the climate may be less effective than had been hoped

“At most, he reckons, the sulphate-injection approach could counteract half of the warming the world is expected to suffer over the next 100 years if carbon-dioxide emissions continue to rise unchecked.

Other options seem even less effective. Encouraging cloud formation over the oceans by spraying seawater into the air would be roughly as helpful as pumping the stratosphere full of particles. Its effects, though, would be geographically patchy. And dumping nutrients such as iron into the sea would be only one-sixth as effective as either sulphate injection or promoting the formation of clouds.

Moreover, effectiveness is only one way to rank the ideas. In theory, a solar shade could provide any amount of cooling, but the researchers estimate that it would have to have an area of 4.1m square kilometres (half the size of Brazil) to offset half the warming expected over the next century, assuming no cuts in carbon-dioxide emissions occur.”

Increasing the amount of biomass – including forests – on Earth will draw some of the accumulated CO2 out of the atmosphere. What it cannot do is correct for the fact that we are releasing additional CO2 every year.

In the end, to stop climate change we need to stop burning fossil fuels altogether. That is the key action. Alongside that, increasing the amount of forested area can help reduce how much temperature change is experienced.

“The thinking that lies behind this is that though a stratospheric veil of microscopic sulphate particles would undoubtedly cool the earth (they have been clearly seen to do so after large volcanic eruptions) its cooling effect would not be exactly equal and opposite to the warming greenhouse gases provide. Cutting down sunlight delivers less cooling in winter, and none at night. It also has its most direct cooling effect on the surface of the planet, while increasing greenhouse gases warms the surface indirectly by warming the atmosphere above it.

These things can average out. One can imagine setting a system in which the net effect of increased greenhouse gases and increased sulphates in the atmosphere kept the average global surface temperature the same. But that would not mean the climate was unchanged in other ways. In particular, the different effects on the surface and the atmosphere mean that the hydrologic cycle would be changed. Various studies with computer models (and experience with volcanoes) suggest that a greenhouse world kept cool with a sulphate veil will see less precipitation, all things being equal, than a non-greenhouse, non-veiled world with the same average temperature. The pattern of precipitation will change, too, as well as the overall amount.”